1. Field of the Invention
This invention is generally related to a method, apparatus and computer program product that adjusts synchronously beams in a multiple beam optical system, and more particularly relates to detecting and adjusting beams in a multiple beam optical system, where the detection and adjustment of beams is performed in a scanning direction between plural beams that are scanned on a photoconductive member.
2. Background of the Invention
To increase the efficiency in image duplicating devices such as copiers, printers and facsimile machines, a speed of scanning optical beams used in conventional image forming processes has recently been improved. Referring to the FIG. 1, one conventional optical system in a laser beam-based image forming apparatus includes a synchronous detecting device shown as photodetector 6. A beam emitted by a light source unit 1 is projected onto the polygon mirror 3 via an optical lens 2. The polygon mirror 3 is rotated at a predetermined angular velocity in a predetermined direction (shown as a counter clockwise direction as indicated by an arrow in FIG. 1). The beam is scanned within a predetermined scanning angle, and the scanned beam enters into a pair of lenses 4a and 4b and a reflector 9 before reaching the photoconductive drum 5. The above-described optical lens system is exemplary of that used in conventional systems.
In order to maintain a predetermined desired image forming starting point of each scanning operation (i.e., scan line) in the scanning direction on the photoconductive drum 5 after the beam has exited the optical lenses 4a and 4b, the beam is detected by the photodetector 6, located near the photo conductive drum 5. A scanning synchronous controller 7 calculates a time (or distance, which is related to the time) difference between a signal corresponding to the predetermined desired image forming starting point on the photoconductive drum 5 and a signal from the beam detected by the photodetector 6. The scanning synchronous controller 7 determines a revision value based on the time, or distance, difference. The scanning synchronous controller 7 also controls the light source unit 1 on the basis of the revision value where image data is used to modulated the light source at a time that corresponds to the beginning of a scan line.
The above-described feedback loop allows the system to maintain the predetermined desired starting point of the beam in the scanning direction on the photoconductive drum 5.
In the above-described system, the polygon mirror 3 has multiple reflecting surfaces. When the polygon mirror 3 is rotated at a high speed, the beam is scanned in a predetermined direction on the surface of the photoconductive drum 5 so as to form an image thereon. The beam is modulated on and off according to image data corresponding to the scan line. Thus, a scanning angle is defined as a relative angular range of the reflecting surface of the polygon mirror 3 about its rotational axis for covering the horizontal scan distance on the photoconductive drum 5.
However, as determined by the present inventor, at the high rotational speed, a number of undesirable effects need to be considered, such as increased noise and heat, shortened driving motor lifetime for the polygon mirror 3, an expensive bearing unit, a need for a high-intensity laser unit, etc. The extent of these problems generally worsen as the rotational speed is further increased. The costs for solving these associated problems are prohibitive, and another approach is therefore preferred.
Rather than increasing the rotational speed of the polygon mirror 3, other systems employ multiple laser beams that are simultaneously scanned so as to shorten an amount of time to form an image on the photoconductive drum. In other words, if three beams were simultaneously used to scan three lines, it would take only one third of the time as compared to a single laser beam. To take advantage of the multiple beams, the beams should be synchronized in the scanning direction and correctly calibrated to begin scanning at predetermined position, such as at the beginning of a scan line.
However, in the conventional multiple laser beam optical system, the scanning start points of each beam never completely coincide in the scanning direction. Therefore, the difference of the scanning start points in each beam needs to be revised using an adjustment method, particularly when each beam does not appear to the detector 6 as being completely separate from other beams.
If the plural beams are sufficiently separated in the scanning direction, the plural beams may be detected with only one photodetector. When each beam is separately distinguishable (e.g., not overlapping) as shown in FIG. 2, the photodetector 6 independently detects each beam. Referring to FIG. 3, the pulses detected by the photodetector 6 are completely separated from each other in time and thus distinguishable from one another.
Synchronization of each beam is performed during a non-image forming period in a scanning operation. In other words, synchronization is performed between where each beam is detected with the photodetector 6 and when an image forming scan line process starts on the photoconductive drum 5. The faster polygon mirror rotates, the shorter this non-image forming period becomes. Therefore, if the time interval between each beam is too wide, it is difficult to carry out the synchronization operation during the above non-image forming period.
Referring to FIGS. 4 and 5, it is seen that when the plural beams are not sufficiently separated in time, the detected signals overlap in time. In this case the scanning synchronous controller 7 may judge the overlapped signals as one signal which makes it difficult for the scanning synchronous controller 7 to properly perform the synchronization operation.
Some conventional multiple beam optical systems have attempted to solve the above problem. One of these systems is disclosed in Japanese Laid Open JP 59-26,005, in which the optical system uses plural photodetectors arranged to receive respective of the beams. The plural photodetectors are step-wisely positioned along the scanning direction and offset in a direction normal to the scanning direction. The above-described optical system carries out a series of steps including irradiating one detector with only one beam, detecting the beam with the photodetector corresponding to that laser beam and turning-off of the respective beams in the order of arrival at the respective photodetector.
However, the present inventor identified that the expense of including plural photodetectors detracts from the value of the system. Furthermore, each beam must be handled separately so as to ensure proper alignment of the source and detectors during final assembly in the optical system, creating a risk that the clearance of the plural photodetectors differs between image forming apparatuses. Referring to FIG. 6, when the above optical system is assembled in the image forming apparatus during final assembly, the clearance of each photodetector needs to be revised on the basis of the shift of each beam in the scanning direction, thereby increasing manufacturing cost.